I think it's because cities aren't very natural for human beings. There are some big upsides to living in the city, but also tons of downsides. It has this weird effect on people. They lose touch with nature (the environment and their own human nature), get bogged down in the grind of being stuck in traffic and working at some boring place all day, surround by generic looking black/grey boxes with windows (modern excuse for "architecture"), surrounded by crime and poverty, etc. It must warp the mind. People fill the void with all kinds of weird, self-destructive, degenerate behaviour.